During the measurement of bioelectric signals (e.g., ECG signals), common-mode interference signals occur as a result of non-ideal measurement inputs of the measurement arrangement. Common-mode interference signals are interference resulting from common-mode signals such as may arise, for example, from the power supply system frequency at 50 Hz.
Common-mode interference signals occur if dissimilar conditions, such as different impedances and capacitances, occur at the two measurement inputs during a differential signal measurement. One example of a conventional measurement arrangement for measuring an electrocardiogram (ECG) is illustrated in FIG. 1.
FIG. 1 shows a circuit arrangement for measuring ECG signals of a patient P in accordance with the prior art. The circuit arrangement includes a first electrode 1 and a second electrode 2, which are in contact with the patient P in such a way that a heart current may flow via the electrodes 1 and 2 to a differential amplifier 4. The differential amplifier 4 includes a first input 5, a second input 6, and an output 7. The first input 5 is electrically connected to the first electrode 1, and the second input 6 is electrically connected to the second electrode 2.
The output signal of the amplifier 4 is communicated to a first signal detection unit 12 that detects the signal amplified by the amplifier 4. The two electrodes 1 and 2 are symbolized by an RC element illustrating the impedance values of the two measurement paths 15, 16. In this case, a first measurement path 15 runs from the contact of the first electrode 1 with the patient P via the first electrode 1 to the first input 5 of the amplifier 4, and the second measurement path 16 runs from the contact of the second electrode 2 with the patient P via the second electrode 2 to the second input 6 of the amplifier 4.
One example of an ECG signal E subjected to common-mode interference as a result of an impedance difference of 500 kohms is shown in FIG. 2. The associated measurement set-up corresponds to the set-up in FIG. 1. In the graph shown, the amplitude A of the ECG signal E in mV is plotted against time t in seconds. The amplitude A of the interference sources is approximately 1.3 mV in the example with an impedance difference of 500 kohms. In this example, there is a strong ECG signal E having an amplitude A of more than 2 mV, but there are also patients P having an amplitude A of only 0.1 mV, which would completely vanish in these interference sources. In the case of larger impedance differences, the amplitude A of the common-mode interference signals rises further and may also reach a multiple of the representation shown.
In principle, common-mode signals (e.g., interference signals) are not concomitantly amplified in a differential measurement arrangement, and so the common-mode signals are suppressed. However, the different impedances of the inputs of the measurement arrangement have the effect that different input signals caused by the same interference signal are present at the two inputs of an amplifier circuit of a differential measurement arrangement, such that the then differential interference signal is amplified together with the actual measurement signal. These common-mode interference signals are very strong in the application on the patient (e.g., a human being or an animal), since the electrode contacts on the patient's skin have a greatly varying quality factor without complex preparation.
An electrode contact on the patient may have impedances of between 10 kohms and several megohms and likewise greatly varying capacitances. As a result, the difference between the impedances and capacitances at two measurement inputs is also in the range of up to several megohms. One example of an ECG signal E subjected to common-mode interference by such an impedance difference of 500 kohms is shown in FIG. 2. In some instances, the impedance differences at the inputs of the measurement arrangement are even higher, such that an evaluation of a measurement signal is hardly possible or may lead to incorrect diagnoses.
By contrast, the total sum of the impedances of the electrode contacts is hardly of importance on account of progress in circuit technology with input impedances of from hundreds of megohms to several gigaohms; it is completely irrelevant to the common-mode interference signals.
A method for suppressing common-mode interference during the measurement of bioelectric signals is not permissible in all cases of diagnosis since the latter in part normatively requires a guaranteed transfer function of the represented measurement data, but that may not be guaranteed by adaptive methods. However, common-mode interference signals are interfering in a diagnostic environment and may lead to incorrect diagnoses.
Hitherto the assessment of signal interference in a diagnostic environment has been incumbent on the physician, who, on the basis of the measurement signal composed of useful signal and interference signal, is to determine himself/herself which signal components are diagnostically significant and which are caused by interference.
In order to minimize the proportion of interference, the medical personnel ought therefore to provide a good and stable electrode-skin contact resistance and thus as far as possible symmetrical measurement paths. However, even with very good contact resistances, freedom from interference is not guaranteed since a perfect symmetry is not possible just on account of tolerances in the measurement set-up. Only few measurement devices carry out an impedance measurement of the electrode contact resistance in order to provide the medical personnel with assistance when attaching the electrodes.